Request keyboard grab ability under Xwayland

We're usually white listed, but let's be a good citizen and formally
request the keyboard grab ability from the compositor.
diff --git a/vncviewer/DesktopWindow.h b/vncviewer/DesktopWindow.h
index 6ec8e1b..c408cb7 100644
--- a/vncviewer/DesktopWindow.h
+++ b/vncviewer/DesktopWindow.h
@@ -73,6 +73,7 @@
   void setLEDState(unsigned int state);
 
   // Fl_Window callback methods
+  virtual void show();
   void draw();
   void resize(int x, int y, int w, int h);